1. Field of the Invention
The present invention relates to a noise suppressing circuit suitable for a video camera image pickup device using a progressive scan CCD, and, more particularly, to a circuit which suppresses noise in a color signal component.
2. Background Art
One example of a conventional image pickup device is described in the following Patent Literature 1. FIG. 11 is a block diagram showing the conventional image pickup device. The output signal of a CCD 1 goes through a CDS (Correlated Double Sampling)/AGC A/D block 2, and a digital signal is applied to a luminance generation LPF 3 and a color generation LPF matrix 5. A luminance signal is generated in the luminance generation LPF 3, and the luminance signal is then subjected to γ correction by a γ processing section 4. Two color signals are generated in the color generation LPF matrix 5, the color signals are then subjected to γ correction by γ processing sections 6-1 and 6-2, respectively, and those color signals are respectively applied to noise suppressing circuits 7-1 and 7-2 to suppress noise in color signals.
FIG. 12 shows a noise feedback IIR (Infinite Impulse Response) filter using a 1H line memory. After the gain of the output signal of a subtracter is halved by a gain controller (which is actually realized merely by bit shifting because the signal is a digital signal), a comparison process section S which passes only a small amplitude is intervened. As shown in FIG. 12, the contents of the comparison operation of the comparison process section S are such that given that the size of the output signal of the gain controller is Nin, in both cases where Nin is greater than a predetermined value +A, and where Nin is smaller than the predetermined value +A, an output signal Nout of the comparison process section S is 0, whereas when Nin is equal to or smaller than the predetermined value +A and is equal to or greater than a predetermined value −A, Nin is output as it is. Without this process, the output of the subtracter is supplied to an adder on the right side and is circulated via the line memory, so that components of plural lines are added regardless of the amplitude, but when the noise suppressing circuits 7-1 and 7-2 are inserted in the color signal system, the color is blurred in the vertical direction. To prevent it, only a small amplitude component, such as noise, in the subtracter output is passed, thereby ensuring noise suppression with less color blurring. According to the prior art, the value of k is fixed, and is a block of a gain k0. Therefore, ½·k0 becomes the gain of the loop part. In FIG. 12, therefore, only when Nin is equal to or smaller than the predetermined value +A and is equal to or greater than the predetermined value −A, the noise component of an output F(y) is suppressed with a feedback amount k with respect to an input F(x) as follows.F(y)={F(y−1)−F(x)}×½·k+F(x)
FIG. 13A shows a coring circuit, as another example of the noise suppressing circuit 7-1, 7-2, which improves the S/N ratio at a low illuminance by clipping or performing non-linear suppression of the level of a chroma signal near a low illuminance region. With the structure in FIG. 13A, a comparison process section T operates as follows, and the noise component of the output F(y) is suppressed with a coring amount m with respect to the input F(x) as shown in FIG. 13B.
When F(x)>m0, F(y)=F(x)−m0
When −m0≦F(x)≦m0, F(y)=0
When F(x)<−m0, F(y)=F(x)+m0
According to the noise suppressing circuit 7-1 and 7-2, when the CCD 1 is an interline type (IL-CCD), upper and lower lines are mixed at the time of scanning, so that a sufficient noise suppression effect (noise component averaging) can be obtained. Patent Literature 1: Japanese Patent Laid-Open Publication No. 2003-189126 (FIG. 1).
Because the PS (Progressive Scan) type CCD (PS-CCD) which is of a type that scans all the pixels scans upper and lower lines without mixing them at the time of CCD scanning, however, the noise component is said to be worse by √2 times (The Journal of the Institute of Television Engineers of Japan: Proposal of Simultaneous Independent Double Line Scanning IL-CCD). To acquire a noise suppression effect equivalent to that of the IL-CCD, therefore, a stronger suppression coefficient (feedback amount k or coring m) should be applied. However, in case of the noise feedback type, stronger application brings about a lasting effect on video images or so, or in the coring process, a phenomenon that coloring is not provided at the time of a low luminance, so that too much application cannot be made.
A solid state image pickup device has a property such that the amount of noise differs according to the output value (brightness), and each scheme mentioned above applies the same suppression to all the pixels, so that if noise suppression of a sufficient intensity is applied to every output value, suppression becomes stronger on pixels with few noise, making the side effect greater, and if suppression is made weaker to avoid it, there may be pixels which cannot bring about a sufficient noise canceling effect.